I kinda skipped about on this thread as I was getting bored, but I always saw, Male looking for female, or every variation of it as a sort of, I play male characters, Looking for female characters. And even on posts where, the few times I've done 1v1's with a dude character, the person had posted " looking for a Dude." or those variations, And I said I can do a dude. They accepted me. 

THis isn't meant to say that some people aren't trying to discriminate people in their rp's but i'm pretty sure the majority are only looking for characters to fill their slots. 

The part where they may be looking for " female roleplayers" could be based on past experiences where a male roleplayer did not do a well job expressing the character. Or it could be based on the presumption that a female roleplayer would know how to play a female character better, due to being one and shit. Though I'm sure they wouldn't kill all discussion if the person interested in the rp can play a female well and is perhaps male. Which is not sexist but just a precaution to make sure only those qualified try for the role. 
I know on one of my very old interest checks I had a " looking for male" on it, cause i only did female characters at the time. Did i actually care/know about the true genders of my partners. Nope. THey could play a dude. I didn't care past that. ( Ahh young book.) 
All I see this as is a matter of personal preference and choice. 

If you don't want to rp with someone because their interest check's title/description happens to state something you disagree with. Good for you. 
If you don't care and can play the part. Good for you.

If you want to specify that you want female partners because of some notion, actual sexism, nervousness, or just for precaution. Good for you.

If you don't care about that sort of stuff. Good for you. 

Why is everyone jumpy about stuff that doesn't really matter in the first place? It's just an interest check.